1. Field of the Invention
The present invention relates to an improvement in a cylindrical lens for a beacon that converges beams of light onto a horizontal surface.
2. Prior Art
A cylindrical lens has been widely used for lighting waterways, etc. With a light source in the center, the cylindrical lens is designed so that it has a focal point in the center. Light emitted from the light source passes through the lens and converges on parallel beams of light at the height of the lens.
Unlike the photographic lens or magnifying lens, resolution or distorted aberrations are not a problem for the cylindrical lens used for this purpose, which is only concerned with the convergence efficiency of light-emission energy.
In order to increase the luminous intensity, the numerical aperture of the lens can be increased, but the diameter of the lens is determined by the focal distance, and in order to increase the numerical aperture, the lens height must be raised. However, the incidence angle from the light source to the lens is limited to a maximum of .+-.60.degree.to 65.degree.. In addition, in order to reduce loss of light passing through the lens, the lens must be thin. In view of these conditions, a fresnel-lens-type cylindrical lens is used. FIG. 4 shows a schematic view of such lens.
So as to design the cylindrical fresnel lens to be thin, the lens includes a main lens part and a multi-layered annular part provided above and below the main lens part, and the light-emitting plane of which correspond to the lens surface and consists of a curved plane requiring precision in forming.
Currently, cast glass has been used as a material for the cylindrical fresnel lens for waterway beacon lighting equipment. However, because of various reasons such as mold precision, casting, mold releasing and cooling process, it is impossible to manufacture a high performance lens.
Recently, high performance cast cylindrical lenses are produced by the use of a plastic injection molding method so as to secure a proper thickness of the cylindrical lens. Either way, molds for the casting require a high precision and are expensive. In the case of plastic lens, a certain volume has to be continuously provided in order to stabilize the quality, thereby resulting in high manufacturing cost.
Demand for waterway beacon lenses is small compared to general commodities. The combination of special attributes of the lens, expensive mold, limited production volume and severe performance requirements have made development of a new lens for a demanded purpose very difficult.
The thinner the lens, the less light is lost through the lens. Therefore, the smaller the pitch between the rings of the fresnel lens, the thinner the lens can be made. Recent improvement in machining precision of molds and progress in plastic materials made a pitch of 0.1 mm possible. Also, if the pitch is made small, sufficient precision can be obtained even if the vertical section of the light-emitting plane is processed linearly, i.e., in a prism shape, instead of a curved line, which makes machining of a mold very easy.